CEPHEIDS

What are Cepheids? Stars that “pulse” and change luminosity Very bright (100,000x luminosity of Sun) Used to measure extreme distances in space Their brightness allows for measurements to distant galaxies

Cepheids- history First discovered by John Goodricke in 1784 Named after this first star (Delta Cephi) in the constellation Cepheus (the King) Was not known at that time the important significance they would play Henrietta Leavitt (1912) studied 500+ Cepheids in the Magellanic Cloud

History- continued Harlow Shapely (1915) charted size of Milky Way using Cepheids Leavitt and Shapely determined that the period of a Cepheid varied directly with its luminosity- (i.e., the brightest Cepheids have the longest periods) Edwin Hubble’s work with Cepheids settled the debate of whether the Milky Way represented the entire universe, or was merely one galaxy of many

Cepheid Characteristics Yellow Super Giants (Classical Cepheids) Drifted off the main sequence into instability zone Pulsation is an actual change in temperature and diameter of the star Typically ~25% change in size Classical Cepheids are 4-20x more massive than the Sun, 100,000x or more brighter pulsating stars pulsating stars

"HR-diag-instability-strip" by Rursus - Own work. HR diagram showing location of Cepheid variables compared to main sequence stars

Classical vs. Type II Cepheids Type II Cepheids (Population II Cepheids) Metal-poor Old stars Smaller than Sun (~1/2 solar mass) Subdivided by period length (1-4 days, 5-10 days, days, 20+ days)

Cepheids Graphs Remember the direct relationship: the longer the period, the brighter the Cepheid

Cepheids Graphs What is the period of the Cepheid in this graph?

Nearest Cepheid Polaris (See location on HR Diagram) Classified as a classical Type I Cepheid Recent evidence has showed Polaris is more than 100 light years closer than previously thought Polaris Closer Than Thought